The invention relates to a cathode sputtering apparatus having
(a) a hollow cathode on the magnetron principle with a cathode base having at least one cooling passage and an inner cylindrical surface for receiving a hollow target having an inner, likewise substantially cylindrical sputtering surface and a cylindrical outside surface. A magnet system externally surrounds the cathode base, with magnet poles for the production of a rotationally symmetrical tunnel of magnetic lines of force closed on the circumference and over the sputtering surface.
(b) at least one anode substantially coaxial with the sputtering surface lies outside of the space surrounded by the sputtering surface.
(c) with a path for the transport of a substrate to be coated runs through the at least one anode.
DE-OS No. 22 43 708 has disclosed such cathode sputtering apparatus in which the plasma produced in operation is confined on six sides, four sides of which are formed by the target and the magnetic lines of force and the two remaining sides are formed by the annular closing of the magnetic tunnel. Magnetron cathodes obey the formula E.times.B, E defining the electrical field and B the magnetic field. The sputtering rate is highest where the lines of force of E and B intersect at right angles and the magnetic lines of force run parallel to the target surface.
In some of the embodiments of hollow magnetrons according to DE-OS No. 22 43 708, largely axis-parallel sections of the magnetic lines of force are used in the area of the sputtering surfaces. Since here the condition E.times.B is optimally fulfilled just about everywhere, the target material is eroded very uniformly. The very high material efficiency that is thus possible, however, is achieved at the cost of the considerable disadvantage that, in order to satisfy the quadrilateral condition, the target has to have flanges made of target material that face radially inward. This makes the target expensive and it is subject to an uneven thermal expansion with additional heat input, because the flanges act to some extent as "heat ribs."
In other embodiments of hollow magnetrons pursuant to DE-OS No. 22 43 708, the quadrilateral condition is satisfied by the fact that the magnetic lines of force are severely curved, issue from the cylindrical or conical sputtering surfaces at one point, and after passing through arcuate paths return at a different point. Thus the condition E.times.B is optimally fulfilled only in the area of circular lines which pass through the culmination points of the magnetic lines of force situated above the sputtering surface, because only in the vicinity of the culmination points do the magnetic lines of force run at least approximately parallel to the target surface. This does permit the production of targets that are geometrically simple since they are hollow-cylindrical, but it has the considerable disadvantage that the material ablation takes place predominantly under the above-mentioned culmination points or lines, so that deep erosion pits are formed in the target on its inner circumference, leading very soon to the failure of the target and thus to a poor material efficiency. The local heat input corresponds to the local sputtering rate, so that the linearly eroded hollow cylindrical targets are subject to a slight barrel-shaped distortion, but one which nevertheless interferes with the heat transfer between the transfer surfaces.
In both cases, therefore, poor target cooling conditions are involved, so that it has been necessary to resort to exposing the target at its outside surface directly to the coolant (water). This, however, leads to sealing problems between the cathode base and the target, and it is to be noted that cathode sputtering processes are performed with vacuums between about 10.sup.-2 and 10.sup.-4 mbar, so that even minimal leakage leads to a collapse of the vacuum and/or to contamination of the products. Changing the target thus becomes extraordinarily difficult and time-consuming.